The present invention relates to an electrophotographic photosensitive material, and more particularly to an electrophotographic photosensitive material possessing a surface protective layer.
In an image forming apparatus such as copier utilizing the so-called Carlson process, an electrophotographic photosensitive material forming a photosensitive layer on a conductive substrate is used.
Since the electrophotographic photosensitive material is repeatedly exposed to electrical, optical and mechanical impacts in the image forming process, a surface protective layer containing binding resin is laminated on the photosensitive layer for the purpose of enhancing the durability to these impacts.
As the binding resin, a thermoset silicone resin is mainly used for improving the hardness of the surface protective layer.
The thermoset silicone resin may be cured by heating only, depending on the conditions, without using catalyst, but a catalyst is generally used for finishing the hardening reaction smoothly and uniformly.
As the catalyst for hardening the thermoset silicone resin, inorganic acids, organic acids, alkalis such as amines, and various materials are generally used, but the following performances are required for the catalyst for hardening the thermoset silicone resin used in the surface protective layer.
(1) Capable of forming a surface protective layer excellent in mechanical strength by hardening.
(2) Not to adversely affect the sensitivity and other properties of electrophotographic photosensitive material.
As the materials having these performances to a certain extent, organic tin compounds such as dibutyl tin dilaurate (DTL) and dibutyl tin dioctate (DTO) have been proposed (see the Japanese Laid-Open Patent No. 60-4945).
However, the surface protective layer hardened by using such organic tin compound is not sufficient in the wear resistance, or the initial sensitivity of the electrophotographic photosensitive material is not sufficient, or the surface potential of the photosensitive material is lowered when exposed repeatedly, and the catalyst remaining on the surface protective layer sometimes adversely affected the photosensitive characteristics.